Only allow one value for each plugin type in plugins flag

[four4fish]

What does this PR do?

Fixes #11449 follow up item : check there is only 1 instance of each type at most in plugin flag

Does your PR introduce any breaking changes? If yes, please list them.

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[kaushikb11]

@four4fish How is this a breaking change? It was never supported.

[four4fish]

@four4fish How is this a breaking change? It was never supported.

@kaushikb11 If user set two checkpointIO in plugins before, we used the later one instead of throw exception. Does this considered breaking change?

[kaushikb11]

@kaushikb11 If user set two checkpointIO in plugins before, we used the later one instead of throw exception. Does this considered breaking change?

IMO, we should throw exception for this case as well.

[four4fish]

@kaushikb11 If user set two checkpointIO in plugins before, we used the later one instead of throw exception. Does this considered breaking change?

IMO, we should throw exception for this case as well.

Checkpoint io duplication check is included in this PR. Just a example to explain why this could be a breaking change

[awaelchli]

While reviewing the PR, also found this malformatted error message:

pytorch_lightning.utilities.exceptions.MisconfigurationException: You have passed Trainer(strategy=ddp) and you can only specify one strategy, but you have passed <pytorch_lightning.plugins.training_type.ddp_spawn.DDPSpawnPlugin object at 0x7fc2bb458310> as a plugin.

it can be triggered by

Trainer(
        strategy="ddp",
        plugins=[DDPSpawnPlugin()]
)

for example. Would be nice follow up :)